Secretion of cholesterol, triglyceride and apolipoprotein A-I by isolated perfused liver from rats fed soybean protein and casein or their amino acid mixtures.

Isolated livers from rats fed soybean protein isolate and casein or amino acid mixtures simulating these proteins were perfused for 4 hours with Krebs-Henseleit buffer containing 0.15% glucose and 25% human red blood cells. Feeding soybean protein as compared with casein resulted in a significant reduction of the secretion of cholesterol, triglyceride and apo A-I. The results agreed well with the responses of serum counterparts reported previously. When amino acid mixture diets were fed, however, no such difference could be demonstrated, though the soy-type amino acid mixture had also been shown to decrease serum cholesterol and apo A-I. The production rate of total ketone bodies was the same, but the ratio of beta-hydroxybutyrate:acetoacetate was significantly higher in rats fed plant protein. The perfusate glucose tended to be higher on soybean protein. These differences were less clear on feeding amino acid mixtures. Neither the rate of bile flow nor the concentration of biliary bile acids and cholesterol were influenced by the type of dietary protein. These observations led us to conclude that soybean protein exerts the cholesterol-lowering action primarily through the regulation of hepatic contribution. The data also suggested that the protein-dependent difference in the concentration of serum cholesterol and apo A-I might not be explained thoroughly by the difference in the amino acid profile alone.